Contact protection using charge storage diodes

ABSTRACT

A diode characterized by a long minority carrier lifetime and supplied with a charging current is connected in parallel with a pair of electrical contacts. When the contacts are opened the diode provides, in effect, a short circuit across the contacts thereby preventing arcing for a predetermined period until the accumulated charge in the diode is depleted.

United States Patent Inventor Sigurd G. Waaben Princeton, NJ.

Appl. No. 817,053

Filed Apr. 17, 1969 Patented Aug. 24, 1971 Assignee Bell Telephone Laboratories incorporated Murray Hill, NJ.

CONTACT PROTECTION USING CHARGE STORAGE DIODES 7 Claims, 3 Drawing Figs.

U.S. Cl 307/136, 317/114, 307/300, 307/319, 307/10 1111. C1 H0111 9/30 Field of Search 307/136, 300,319; 317/114 PRIMARY COIL CHARGE STORAGE DIODE References Cited UNITED STATES PATENTS 3,145,700 8/1964 Root 200/44 X 3,43 I ,466 3/1969 Watanabe et al. 317/1 1.4 3,075,124 l/1963 Bagno 307/300 X Primary Examiner-Robert K. Schaefer Assistant Examiner-H. J. Hohauser Atl0rneysR. J. Guenther and Kenneth B. Hamlin ABSTRACT: A diode characterized by a long minority carrier lifetime and supplied with a charging current is connected in parallel with a pair of electrical contacts. When the contacts are opened the diode provides, in effect, a short circuit across thecontacts thereby preventing arcing for a predetermined period until the accumulated charge in the diode is depleted.

SECONDARY COIL .3 PRIMARY CIRCUIT iBREAKER CONTACTS PATENTED AUS24I97I 3,601. 622

FIG.

' PRIMARY II L+ COIL SQR T l7 (I8 20 II M [A CHARGE I) STORAGE l6 DIODE PRIMARY I l2- /VV\F' CIRCUIT /21 IBREAIIER CONTACTS FIG. 2

SEPARATION BETWEEN CONTACTS OF SWITCH l2 Lu t.) E MAxIMuM SEPARATION FOR WHICH 5 ARCING OCCURS I l TIME 1] FIG. 3

3| 30 SK II.

H'I I T 32 33 /N l/EN TOR 5.6. WAABEN A TTORNEV CONTACT PROTECTION USING CHARGE STORAGE DIODES BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to electrical contact protection networks and more particularly to networks for preventing arcing between circuit breaker contacts such as in an automobile ignition system.

2. Description of the Prior Art It is customary in the prior art to connect a capacitor across a pair of electrical contacts to provide protection against arcing when the contacts are opened. For example, in an automobile ignition system, a capacitor is normally connected in parallel with a pair of breaker contacts which are included in the primary coil circuit. When the opening of the breaker contacts interrupts the current in the primary circuit, which causes a collapse in the magnetic field associated with the primary coil, the capacitor performs the dual functions of regulating the current in the primary coil and protecting the contacts from the intense are which can result. Although such a capacitor provides adequate contact protection when the peak voltage in the primary coil is induced at a moderate rate, it is not effective when the primary coil voltage rise time becomes extremely short. In fact, the demands placed upon the capacitor are conflicting since a smallercapacitor, which inherently provides less contact protection, is required to decrease the voltage rise time in the primary coil.

' It would be advantageous to provide a contact protection network for an automobile ignition system which does not impose undue restraint upon the rise time of the voltage induced in the primary coil. If this were done it would be possible to design the system using a smaller capacitor in the primary coil thereby improving the overall efficiency of the ignition system. Less power would then be lost when the engine is operated at ,high speed and spark plug lifetime would be increased.

Summary of the Invention against arcing while permitting the voltage rise time in the primary circuit of the ignition system to be shortened.

These and other objects of this invention are attained in an illustrative switch contact network comprising a charge storage diode, which is a diode having a long minority carrier lifetime, connected in parallel with the switch contacts and poled such that it accumulates charge while the switch contacts are in the closed position. When the switch contacts are opened the charge storage diode provides a momentary short circuit across the switch contacts and, accordingly, no arcing occurs. This phenomenon is a result of the charge accumulated in the diode while the switch contacts are in the closed position and persists until such charge is depleted. The time period between the opening of the switch contacts and the depletion of charge stored in the diode is determined, in part,

by the minority carrier lifetime of the diode. If a diode of appropriate minority carrier lifetime is-selected, the switch contacts are substantially separated by the time charge depletion occurs and thus there is no arcing between the switch contacts.

2 BRIEF DESCRIPTION OF THE DRAWING THe objects and features of this invention will become fully apparent by consideration of the following detailed description and drawing in which:

FIG. 1 is a schematic diagram of a contact protection network in accordance with this invention connected in an automobile ignition system;

FIG. 2 is a graph showing two curves useful in network shown in FIG. 1; and

FIG. 3 is a schematic diagram of another contact protection network constructed in accordance with the invention.

describing the DETAILED DESCRIPTION An illustrative embodiment of a primary circuit of an automobile ignition system having a contact protection network constructed in accordance with this invention is shown in FIG. 1. The primary circuit is supplied by battery 11 and controlled by manual ignition switch 12 connected adjacent the negative terminal of battery 11. The positive terminal of battery 11 is connected through conduction path 21 to one side of primary coil 19. Associated with primary coil 19 is secondary coil 20 which is in direct electrical contact with the distributor and spark plugs (not shown) of the automobile. TI-Ie other side of primary coil 19 is connected to the negative terminal of battery 11 through point 22, breaker contacts 13 and manual switch 12, thus completing the primary circuit.

Breaker contacts 13, which are of a normal variety commonly used in automobile ignition systems, are connected in parallel with a conduction path containing capacitor 14 and resistor 15. Capacitor l4 and resistor 15 are of varieties commonly used at the present time to provide contact protection for the breaker contacts and to regulate the reduction of current in the primary coil of automobile ignition systems. It will become apparent that capacitor 14 and resistor 15, while providing some contactprotection for breaker contacts 13, are of only secondary importance in this regard in the configuration shown in FIG. 1.

Contact protection for breaker contacts 13 is provided by a conduction path connected at one end to point 22 and at the other end to a plate of battery 12 having an electrical potential sufficient for conducting a forward current through charge storage diode 18. Connected into this conduction path is the parallel combination of resistor 17 and diode 16, which is poled in a direction toward battery 11. Resistor 17 regulates current flowing from battery 11 to point 22, and diode 16, which is a standard diode, shunts resistor 17 when current flows from point 22 to battery 11. This conduction path also includes charge storage diode l8 serially connected therein and poled toward point 22.

Charge storage diode 18 is any one of a varietyof diodes which are characterized by a long minority carrier lifetime. Such a diode stores charge near its P-N junction when it is exposed to a forward current. If a reverse bias is applied to a previously charged diode, essentially no impedance is provided by the diode for a predetermined period of time, said period being related to the minority carrier lifetime of the diode. During this period the charge stored in the diode diminishes. When the charge is completely depleted, the diode acts as a standard diode and thus provides effectively an open circuit to a current in the reverse direction. If a forward current is again directed through the diode, charge is again stored and the diode again passes a reverse current for a predetermined period of time.

The operation of the configuration shown in FIG. 1 is now described. Assume initially that manual switch 12 and breaker contacts 13 are closed. Battery 11 supplies a current to the primary circuit through conduction path 21, primary coil 19,

A current also flows from battery 11 through resistor 17 and charge storage diode 18 to point 22. From point 22 this current proceeds through breaker contacts 13 and switch 12 to the negative terminal of battery 11. Accordingly, a charge is stored in charge storage diode 18 during this time.

When breaker contacts 13 are opened, an induced surge of voltage occurs in primary coil 19 as the magnetic field collapses. This results in an induced voltage in secondary coil 20 which is transmitted through the distributor to the spark plugs (not shown) of the'automobile. With breaker contacts 13 open the induced voltage from primary coil 19 causes the current in the primary circuit to be diverted at point 22 through charge storage diode 18 and standard diode 16. Since charge has previously been accumulated in charge storage diode 18, diode 18 offers almost no impedance to a reverse current flowing from primary coil 19 through point 22. Similarly, standard diode 16 offers no impedance since the current is flowing in a forward direction therethrough. Thus, the only voltage across breaker contacts 13 during this period is the minimal potential difference provided by battery 11. In effect, breaker contacts 13 are short-circuited by the path extending from point 22 through charge storage diode l8, and thus no arcing occurs.

This situation continues for a predetermined length of time, that is, as long as there are minority carriers in charge storage diode 18. If a proper variety of charge storage diode is selected, breaker contacts 13 will be sufficiently separated by the time charge depletion occurs in the diode so that no arcing will occur.

The operation of this contact protection network is better understood by reference to FIG. 2 in which curve 21 portrays the separation of contacts 13 with respect to time and curve 22 portrays the maximum separation at which arcing can occur between contacts 13. At time T the primary circuit is broken through contacts 13 and thereafter the gap between contacts 13 increases linearly with respect to time. Then, for a period until time T breaker contacts 13 are effectively shortcircuited through charge storage diode 18 and diode 16 as described above. Thus, during this period no arcing occurs between breaker contacts 13 as is appreciated from curve 22. At time T the charge stored in charge storage diode 18 is depleted and the reverse current therethrough is blocked. Capacitor 14 then begins to charge through resistor 15 and the voltage across breaker contacts 13 begins to increase. With a voltage across breaker contacts 13, arcing therebetween becomes possible and accordingly curve 22 rises to a maximum level from the horizontal axis. However, as indicated in FIG. 2, contacts 13 are widely separated at this time and no arcing occurs even at the point where curve 22 reaches its maximum value.

It is thus apparent that the use of charge storage diode 18 to provide contact protection provides greater freedom in the design of automobile ignition systems. Previously, the size of the capacitor connected in parallel with the breaker contacts was determined largely by the necessity of preventing arcing between the breaker contacts. Thus, if the capacitor was too small, the voltage would increase rapidly across the breaker contacts and arcing therebetween would occur. Since the size of the capacitor also determines the rate of voltage increase in the primary coil, a limitation was imposed upon the acceptable voltage rise time in the primary coil. Capacitors which were small enough to provide a rapid rise time in the primary coil would also cause arcing between the breaker contacts. The use of a charge storage diode, however, to provide contact protection permits far greater flexibility in the selection of a capacitor to be connected in parallel with the breaker contacts. Since the capacitor does not begin to charge until a predetermined period has elapsed after the opening of the breaker contacts, smaller capacitors, which in previous arrangements would have resulted in arcing between the breaker contacts, can be used. Thus, faster voltage rise times may be obtained in the rimary coil.

In addition, the re uction in the size of capacitor 14 decreases the quantity of charge stored therein and thus minimizes the chance that arcing will occur when breaker contacts 13 are closed.

It is apparent that this invention is not limited to use in connection with automobile ignition systems but instead has wide applicability and may be used in any situation where it is desired to prevent arcing when a pair of electrical contacts are opened. FIG. 3 shows the general use of a contact protection network constructed in accordance with this invention to protect a pair of contacts 31. Contacts 31 are connected between a positive source 34 and ground through conduction path 30. Conduction path 30 may include various kinds of electrical elements and may have a net inductive or capacitive characteristic. Connected in parallel with contact 31 are battery 33 and charge storage diode 32, which is poled in a forward direction with respect to battery 33.

THis circuit operates similarly to the circuit shown in FIG. 1. While contacts 31 are in closed position, battery 33 provides a forward current through charge storage diode 32. Thus, in the manner described above, charge is stored in charge storage diode 32. When contacts 31 are opened, charge storage diode 32 provides in effect a short circuit across contacts 31 and thereby prevents arcing until contacts 31 are completely separated.

I claim:

1. An electrical contact protection network comprising a pair of electrical contacts, a semiconductor element characterized by a long minority carrier lifetime connected in parallel with said contacts, and means for supplying a current through said semiconductor element while said contacts are in the closed position such that charge is stored in said element.

2. An electrical contact protection network in accordance with claim 1 wherein said semiconductor element comprises a diode.

3. An electrical contact protection network in accordance with claim 2 wherein said means for supplying current comprises a voltage source for supplying a forward current through said diode such that when said contacts are opened said diode presents effectively no impedance for a predetermined length of time.

4. An automobile ignition system comprising a battery, a primary coil, and a pair of breaker contacts connected in series wherein the improvement comprises a semiconductor element connected in parallel with said contacts,.and means for supplying current through said semiconductor element while said contacts are in the closed position such that charge is stored in said element.

5. An automobile ignition system in accordance with claim 4 wherein said semiconductor element comprises a charge storage diode which is poled in opposition to the voltage surge produced by said coil when said contacts are opened and wherein said means for supplying current is a battery which forward-biases said diode.

6. An automobile ignition system in accordance with claim 5 additionally comprising the parallel combination of a resistive element and a standard diode, said combination connected in series with said charge storage diode.

7. An automobile ignition system in accordance with claim 6 additionally comprising the series combination of a capacitor and a resistive element connected in parallel with said contacts. 

1. An electrical contact protection network comprising a pair of electrical contacts, a semiconductor element characterized by a long minority carrier lifetime connected in parallel with said contacts, and means for supplying a current through said semiconductor element while said contacts are in the closed position such that charge is stored in said element.
 2. An electrical contact protection network in accordance with claim 1 wherein said semiconductor element comprises a diode.
 3. An electrical contact protection network in accordance with claim 2 wherein said means for supplying current comprises a voltage source for supplying a forward current through said diode such that when said contacts are opened said diode presents effectively no impedance for a predetermined length of time.
 4. An automobile ignition system comprising a battery, a primary coil, and a pair of breaker contacts connected in series wherein the improvement comprises a semiconductor element connected in parallel with said contacts, and means for supplying current through said semiconductor element while said contacts are in the closed position such that charge is stored in said element.
 5. An automobile ignition system in accordance with claim 4 wherein said semiconductor element comprises a charge storage diode which is poled in opposition to the voltage surge produced by said coil when said contacts are opened and wherein said means for supplying current is a battery which forward-biases said diode.
 6. An automobile ignition system in accordance with claim 5 additionally comprising the parallel combination of a resistive element and a standard diode, said combination connected in series with said charge storage diode.
 7. An automobile ignition system in accordance with claim 6 additionally comprising the series combination of a capacitor and a resistive element connected in parallel with said contacts. 